Both the alternative-energy and flat-panel display markets have a need for high-quality, flexible substrates on which to produce highly crystalline semiconductor thin films.
The current solar cell (i.e., photovoltaic) market relies on technology that has been essentially unchanged for decades. Over 90% of the market is served by crystalline silicon (Si), either single-crystal or polycrystalline, with average conversion efficiencies of 12-20%. The costs of crystalline Si devices are high due to high-cost production methods and high demand for the raw materials in competition with the semiconductor electronics industry. Si devices must also be quite thick to achieve these efficiencies, consuming significant quantities of material. The remaining 10% of the market is served largely by thin-film structures based on amorphous Si, CdTe, or copper-indium-gallium-selenide (“CIGS”) that are cheaper to produce but have energy conversion efficiencies below 10%. Amorphous Si efficiencies also degrade with time.
Higher conversion efficiencies, over 30%, have been demonstrated for thin film multi-junction devices based on III-V semiconductors such as GaAs. However, their production costs are very high since these devices are most advantageously grown on single-crystal germanium (Ge) or GaAs wafers costing over $10,000 per square meter.
Emerging low-cost photovoltaic technologies include ribbon-grown Si, polymeric/organic films, and nanotechnology-based approaches. None of these new solutions fully addresses the market needs for increased production volume, increased efficiency, and lower cost per watt generated.
A useful substrate for the growth of high efficiency semiconductor films (e.g., III-V semiconductor films) preferably enables the growth of low-defect films (similar to those formed on single-crystal wafers) but at much lower cost and with higher area. Flexibility is a useful attribute. The substrate is also preferably chemically compatible with both the semiconductor material and with the semiconductor process environment. These demanding attributes restrict the number of materials that may effectively be used for this application.
The ability to produce polycrystalline metals with crystallographic orientation (e.g., “biaxial texture”) approaching single-crystal quality in pure metals has been known since the 1940's. Practical applications of such texture control have included the production of aluminum sheet with textures that enhance the production of cans. Most commercial uses of sheet materials avoid texture, however, because the properties of sheet materials are more isotropic in its absence.
Face-centered cubic (fcc) metals, some body-centered cubic (bcc) metals, and some alloys based on fcc metals may be useful as substrate materials, as they may be biaxially textured using well-known rolling-deformation and annealing processes. A well-known texture in fcc metals and alloys is the “cube texture,” in which the c-axis of each of the substrate grains is substantially perpendicular to the substrate surface, and the a-axes align primarily along a length direction. Under controlled rolling and annealing processes, these deformation-textured metals may possess biaxial texture approaching that of single crystals.
Nickel (Ni) is one fcc metal that may be made into thin foils with a well-defined cube texture using a rolling and annealing process. Prior work has shown that oxide intermediate layers may be deposited on a biaxially textured Ni surface using conditions under which nickel oxide is not stable, but where the intermediate layer (for example, CeO2 or Y2O3) is stable, allowing the oxide to inherit the texture of the underlying Ni foil, i.e., form epitaxially thereon. The high-purity Ni required to achieve good biaxial texture is expensive and Ni is mechanically weak following the typical annealing heat treatment used to form the cube texture.
For these reasons, Ni alloys and other alloys have been developed to make stronger, non-magnetic biaxially textured foils. These alloys often contain alloying elements such as tungsten (W), molybdenum (Mo), vanadium (V), or chromium (Cr) in small controlled amounts. Relatively pure copper (Cu) may also be processed to produce a high-quality cube texture. Commercial grades of Cu with relatively low oxygen content and relatively low content of substitutional and interstitial elements have been of particular utility. In addition, prior work has shown that a wide range of Ni—Cu alloys may also be processed to produce high-quality cube textures.
Epitaxial films of other materials such as metals, oxides and nitrides can be grown on the biaxially textured foil. As used herein, “epitaxial” means that the crystallographic orientation of the deposited film is derived from and directly related to the crystallographic orientation of the underlying template.
Unfortunately, the existing deformation-textured foil approach is frequently not commercially viable for the deposition of semiconducting films necessary for high-performance optical and photovoltaic devices, e.g., Si, Ge, GaAs, InP, and related alloys and compounds. One promising approach utilizing Cu or Cu—Ni alloy textured foils is described in U.S. Patent Application Publication No. 2007/0044832A1, the entire disclosure of which is incorporated by reference herein. However, such foils may be incompatible with conventional semiconductor processes. Both Cu and Ni form arsenide and silicide phases when exposed to As- or Si-containing gases at typical processing temperatures above approximately 350° C. The formation of these phases causes a significant increase in volume, embrittles the foils, and renders the foils largely unusable for subsequent processing and for most applications.
One potential solution to the above-described problem appears in U.S. Patent Application Publication Nos. 2009/0117679 and 2009/0114274. Therein, a buffer layer inert to semiconductor precursors “inherits” the texture of the metal foil after formation thereon. The metal foil is then removed before formation of one or more semiconductor layers upon the buffer layer. While this approach addresses the semiconductor process incompatibility issue, the grain size and crystalline orientation of the buffer layer and semiconductor layer(s) are constrained to match those of the original textured metal foil. For many devices and applications, different (e.g., larger) grain size and/or texture are preferred. For example, compound semiconductor films such as GaAs are preferably formed on non-polar substrates (e.g., Ge) having a crystalline orientation tilted away (“off-axis”) from a low-index crystallographic plane. When formed on such substrates, the compound semiconductor films advantageously contain no (or a few closely confined to the growth interface) anti-phase boundaries that may compromise the quality and efficiency of subsequently formed devices. In contrast, compound semiconductor films formed on non-polar substrates having a low-index crystallographic orientation (“on-axis”), e.g., (001) orientation, contain many deleterious anti-phase boundaries. Conventional methods utilizing cube-textured metal films are generally incapable of producing semiconductor films thereon having tilted crystallographic orientations; the cube texture represents a low-energy state for the growing films, and there is no driving force to form tilted orientations in such layers.
Thus, while considerable progress has been made in the use of biaxially textured foils in the production of inexpensive and/or flexible semiconductor thin films and devices, there is a need for processes and structures incorporating larger grain size and crystalline orientations different from the conventional cube texture.